The present invention generally relates to a mount for high speed rotating machinery and more specifically, to a resilient mount that may migrate rotor critical speeds of high speed rotating electrical machinery out of the rotor operating speed range.
The future direction of aerospace quality electric power systems is towards higher power, higher speed, lighter weight, variable frequency electric generators and starter generators. Variable frequency generators rotate throughout a range of speeds within an operating speed range. For a high speed aerospace generator, the operating speed range may typically be from about 10,800 to about 24,000 revolutions per minute (rpm). Potentially large centrifugal forces can be imposed on the rotors of generators operating at such speeds. The generator rotors must be precisely balanced to avoid vibration which may lead to deviation of the rotor shaft axis from its intended axis of rotation. Practically achieving and maintaining this precision balance can be difficult due to variations in the manufacture and assembly process of generators.
The amplitudes of vibrations resulting from rotor out of balance can be significant if the rotor's rotational speed reaches its resonance speed, or a multiple of its resonance speed. Such speeds are generally referred to as ‘critical speeds’. Rotor critical speed and machine response is a function of the rotor mass, the distribution of that mass, the flexibility of the shaft, the bearing support locations and the stiffness of the rotor, bearings, housing and interface between the housing and the bearings.
Typical aerospace generators and starter generators employ rolling element bearings which have very high stiffness which may allow very little rotor variations from the rotor's intended axis of rotation. These rolling element bearings, because they have very high stiffness, places the first critical speed slightly above the maximum operating speed. As the rotor approaches the critical speed, unbalance load increases significantly resulting in large bearing loads and vibration. If an unbalanced rotor is rotating for prolonged periods of time at one of its critical speeds, it may be damaged, even catastrophically. If one of the rotor critical speeds is below the operating range, unbalance loads will be low while passing through it. Once above the first critical speed, the rotor will rotating about its mass center, resulting in low bearing loads and vibration.
U.S. Pat. No. 5,357,547, issued to Obermeyer et al., describes vibration damping of a tubular member. The vibration damper uses an annular sleeve which is attachable to the inside surface of a guide thimble tube which is sized to surround the rotating instrumentation tube. Dimples are attached to the interior wall of the sleeve for radially supporting the instrumentation tube. The wall of the sleeve has a flexible spring member which is formed from the wall for biasing the instrumentation tube into abutment with the dimples. Flow-induced vibration of the instrumentation tube will cause it to move out of contact with the dimples and further engage the spring member, restraining further movement of the instrumentation tube. The Obermeyer vibration damping method may be useful for instrumentation tubes in nuclear power reactor pressure vessels, however, this method may not be particularly useful in aerospace applications, including damping vibration in high speed rotating electrical machinery.
As can be seen, there is a need for an improved resilient mount for rotating electrical machinery that may be easy to manufacture and that may impart the desired stiffness between a bearing and its housing to avoid rotor critical speeds.